In recent years, piezoelectric vibrators using crystals or the like have been used in mobile phones or personal digital assistants, as a time source or a timing source such as a control signal, or a reference signal source and the like. Various types of piezoelectric vibrators are known, but an SMD type piezoelectric vibrator is known as one. With this type of piezoelectric vibrator, generally, a three layer structure type, in which piezoelectric substrates formed with a piezoelectric vibrating reed are bonded so as to be pinched from above and below by a base substrate and a lid substrate, is known. In this case, the piezoelectric vibrator is received in a cavity (a sealing chamber) formed between the base substrate and the lid substrate. Furthermore, recently, a two layer structure type has also been developed instead of the three layer structure type.
This type of piezoelectric vibrator has the two layer structure, in which the base substrate and the lid substrate are directly bonded to each other and the piezoelectric vibrating reed is accommodated within the cavity formed between both substrates.
The piezoelectric vibrator of a two layer structure is excellent in that thinning can be promoted as compared to the three layer structure and is preferentially used. As the two layer structure type piezoelectric vibrator, a piezoelectric vibrator, in which the piezoelectric vibrating reed is electrically connected to an external electrode formed on a base substrate using a conductive member formed so as to pass through the base substrate, is known (see Patent Documents 1 and 2).
However, as shown in FIGS. 26 and 27, a piezoelectric vibrator 200 includes a base substrate 201 and a lid substrate 202 which are anode-bonded to each other via a bonding film 207, and a piezoelectric vibrating reed 203 sealed in a cavity C formed between both substrates 201 and 202.
The piezoelectric vibrating reed 203 is, for example, a tuning fork type vibrating reed, and is mounted on an upper surface of the base substrate 201 in the cavity C via a conductive adhesive E. The base substrate 201 and the lid substrate 202 are insulation substrates that are formed of, for example, ceramic, glass or the like. On the base substrate 201 of both substrates 201 and 202, a through hole 204 passing through the substrate 201 is formed. Moreover, in the through hole 204, a conductive member 205 is buried to block the through hole 204. The conductive member 205 is electrically connected to the external electrode 206 formed on the lower surface of the base substrate 201 and is electrically connected to the piezoelectric vibrating reed 203 mounted in the cavity C.
However, in the above-mentioned two layer structure type piezoelectric vibrator, the conductive member is formed by filling a conductive paste (Ag paste, Au—Sn paste or the like) in the through hole. At this time, in order to reliably block the through hole to maintain the airtightness in the cavity and electrically connect the piezoelectric vibrating reed with the external electrode to secure a reliable conductivity, there is a need to stably fill the conductive paste in the through hole.
For that reason, the formation of the through hole becomes a crucial process.
Generally, as a method of forming the through hole, a method of mechanically drilling using a drill, a method of drilling by laser irradiation, a method of drilling by sand blasting or the like are known.
However, when the through hole is formed using the drill, since the formation state of the through hole depends on the condition (sharpness or the like) of the drill, an irregularity in quality can occur easily. Furthermore, an inner peripheral surface can be easily roughened, whereby it is difficult to finish a flat surface. For that reason, it is difficult to stably fill the conductive paste and the reliability of the airtightness deteriorates. In addition, since a plurality of through holes is usually formed in a wafer step, in the case of using the drill, it takes a long time and is inefficient.
Furthermore, in the case of forming the through hole using a laser beam, since it is affected by a laser beam and a deformed layer is generated in the inner peripheral surface, it is an undesirable method.
Moreover, in the case of forming the through hole by sand blasting, inevitably, the inner peripheral surface is easily roughened depending on the manufacturing method and it is difficult to finish the inner peripheral surface to a flat surface. For that reason, it is difficult to stably fill the conductive paste and the reliability of the airtightness deteriorates.
Herein, as another method of forming the through hole, as shown in Patent Document 1, a method of forming the through hole by press molding using a forming mold while heating the base substrate is known. According to the method, it is possible to accurately form through holes in a uniform quality at a time and finish the inner peripheral surface to a flat surface. Thus, the conductive paste can stably be filled and the reliability of the airtightness can be secured.
Accordingly, this method is superior to other methods in the formation of the through hole.    [Patent Citation 1] JP-A-2002-124845    [Patent Citation 2] JP-A-2006-279872
However, in the method of forming the through hole by press molding, the following problem remains.
Firstly, after the press molding, when separating the forming mold and the base substrate from each other, there is a chance that a pin of the forming mold for forming the through hole is not easily pulled out, whereby the pin can be deformed or bent. Furthermore, since the pin is not easily pulled out, there is a chance that the load is applied to the through hole side and a scratch is generated in the inner peripheral surface of the through hole, whereby a decline in quality occurs.
Furthermore, since the pin of the forming mold is formed in the shape of a taper in which the diameter is gradually narrowed toward the front end thereof to facilitate the press, the through hole is also formed in the shape of a taper. Thus, the diameter of the opening of the through hole can easily become larger as compared to the case of the straight method. Thus, it is difficult to miniaturize the through electrode itself.
However, along with the recent miniaturization of electronic equipment, even with regard to the piezoelectric vibrator mounted on various electronic equipment, further new miniaturization is required in the future. However, when the miniaturization of the through electrode itself is difficult, it is difficult to miniaturize the size of the piezoelectric vibrator, and it has been difficult to respond to the above-mentioned needs.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a piezoelectric vibrator that can be manufactured by the press molding without affecting the pin of the forming mold and that has a through electrode having a high airtightness and in which miniaturization is promoted.
Furthermore, another object thereof is to provide an oscillator, electronic equipment and a radio-controlled timepiece having the piezoelectric vibrator, and a method of manufacturing the piezoelectric vibrator.